Make Signal reading["value"] and value be the same

[coretl]

After bluesky/bluesky#1800 we have a Reading[T] where value is of type T.
After #522 we will have SignalBackend.get_value() and SignalBackend.get_reading()["value"] being the same for all types.
This ticket is to remove the possibility for them to be different.

Change

ophyd-async/src/ophyd_async/core/_utils.py

Line 29 in 406ac08

ReadingValueCallback = Callable[[Reading, T], None]

to

ReadingCallback = Callable[[Reading[T]], None]

ophyd-async/src/ophyd_async/core/_signal_backend.py

Line 45 in 406ac08

def set_callback(self, callback: Optional[ReadingValueCallback[T]]) -> None:

to

def set_callback(self, callback: Optional[ReadingCallback[T]]) -> None:

ophyd-async/src/ophyd_async/core/_signal.py

Lines 140 to 146 in 406ac08

	def _callback(self, reading: Reading, value: T):
	self._signal.log.debug(
	f"Updated subscription: reading of source {self._signal.source} changed"
	f"from {self._reading} to {reading}"
	)
	self._reading = reading
	self._value = value

to

def _callback(self, reading: Reading[T]): 
    self._signal.log.debug( 
        f"Updated subscription: reading of source {self._signal.source} changed" 
        f"from {self._reading} to {reading}" 
    ) 
    self._reading = reading 
    self._value = reading["value"]

Then tidy up all the converters so that they produce reading, rather than reading, value, which means the Converter.value can probably disappear